Release of VPL Jail System 2.3


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The VPL-Jail-System (VPL execution server) serves an execution sandbox for the VPL Moodle plugin. This sandbox provides interactive execution, textual by xterm and graphical by VNC, and non-iterative execution for code evaluation purpose.

For more details about VPL, visit the VPL home page or the VPL plugin page at Moodle.



The of VPL-Jail-System is an open software execution system and require a specific environment.


Software requirements

The VPL-Jail-System 2.3 requires a Linux O.S. and has been tested on Debian, Ubuntu and CentOS.

O.S. Version Results
Ubuntu 18.04 32b/64b Compatible
Ubuntu 16.04 32b/64b Compatible
Debian 9 32b/64b Compatible
CentOS 7 64b GUI programs not available
CentOS 6 Incompatible


Hardware requirements

The system has been developed to offers immediate and interactive execution of student's programs. This means that the system can attend multiple-executions simultaneously.

The hardware required to accomplish this task depends on the number of simultaneous executions at a time, the requisites of the program, and the programming language used. For example, a PHP Web program may requires a huge amount of RAM, especially for the Web Browser execution, but a Python program may need ten times less of RAM.

Our experience is that a machine with only 2Gb of RAM and 2 cores can support a class with 50 students online using Java (Non-GUI). If you are conducting an exam the hardware required may be tripled. Possibly the critical resource may be the RAM. If the system exhausts the RAM the O.S. will start swapping and the throughput will decrease drastically. Our tests indicate that the 32-bit O.S. uses less memory and CPU than the 64-bit version. Remember that you can add (or remove) VPL-Jail-systems to a VPL online.




Selecting the hardware

The recommended option is using a dedicated machine. If you can not use a dedicated machine try using a Virtual Machine e.g. using VirtualBox. This will aisle and limit the resources used by the service. If you decide to use other services in the same machine that the use of resources by VPL-Jail-System may decrease the performance of the others service. Although no security breach has been reported, notice that the nature of the service (execute external code) leads to an intrinsic threat.


Preparing the system

Install a Linux O.S. as clean as possible. If you have enough resources you can install a GUI interface. Stop any service that you don't need as a web server, ssh server, etc. If the O.S. has a firewall, you must configure it (or stop it) to give access to the only two ports needed by the VPL-Jail-System. If you use automatic updates you must restart the VPL-Jail-System to take into account the update. You can use cron to automate this process.


Getting VPL-Jail-System

VPL-Jail-System is distributed only as source files. You must get the source package from eg. using


or from the github repository, generating the package with

make distcheck


Running the installer

After getting the package you must decompress it and run the installer.

tar xvf vpl-jail-system-[version].tar.gz
cd vpl-jail-system-[version]

The "./install-vpl-sh" must be run as root.

Follow the instructions and wait for the necessary downloads. The installation script will try to install the development software commonly used.

The installer will ask you about:

  • If you want that the installer creates a self-signed SSL certificate.
  • (updating) If you want to replace the configuration file with a fresh one.
  • If you want to install different compilers and interpreters.


Updating VPL-Jail-System

If you want to update VPL-Jail-System follow the same steps that the first installation. The installer will update the current version.


Removing VPL-Jail-System

Run uninstall-sh of the current version.



After installing the VPL-Jail-Service, the service will be started with a default configuration. If you want to change the configuration you must edit the file /etc/vpl/vpl-jail-system.conf.

After configuration changes, you must restart (as user root) the service to use the new configuration values.

service vpl-jail-system restart


Main configuration parameters

  • PORT. Socket port number to listen for http and ws connections. The default value is 80
  • SECURE_PORT. Socket port number to listen for https and wss connections. Default value 443
  • URLPATH. Act as a password, if no matches with the path of the URL request then it's rejected. The default value is "/".
  • LOGLEVEL. This value goes from 0 to 8. Use 0 for minimum log and 8 for the maximum log. Level 8 doesn't remove the prisoners' home directory. IMPORTANT: Do not use high loglevel in production servers, you may get low performance. The default value is 0.



You can check the availability of your execution server using the URL

http://server:PORT/OK and https://server:SECURE_PORT/OK

where server is the name of your execution server. The system must return a page with OK



You can obtain a detailed log of the execution process by changing the log level at the configuration file. Commonly The logs will be written to "/var/log/syslog".


Adding the VPL-Jail-System to VPL

The URL of the service in the general module configuration or in the local execution server settings of your Moodle server is

http://server:PORT/URLPATH or https://server:SECURE_PORT/URLPATH

:PORT and :SECURE_PORT can be omitted if using the standard ports.


Changes from the 2.2 to 2.3 version

The main new of the 2.3 version is the change of file system used to replicate root directory on jail. This version includes some minor fixes and is compatible and interchangeable with the previous one.

The replication of the root file system is done with overlayfs, allowing to adapt the replica to the needs of the VPL-Jail-System easily and safe. To accelerate the execution and limit the file system changes, the users' home directory has been mounted as a tmpfs. Also the possibility of mounting the replica allowing SETUID has been added.

The use of the tmpfs removes the need of the "" script.

The new parameters to control these new features are:

  • USETMPFS. This switch allows the use of tmpfs for "/home" and the "/dev/shm" directories. Changing this switch to "false" can degrade the performance of the jail system. To deactivate this option use USETMPFS=false. The default value is USETMPFS=true.
  • HOMESIZE. This option set the size of the "/home" directory. The default value is 30% of the system memory. This option is applicable if using tmpfs file system for the "/home" directory.
  • SHMSIZE. This option set the size of the "/dev/shm" directory. The default value is 30% of the system memory. This option is applicable if using tmpfs file system for the "/dev/shm" directory.
  • ALLOWSUID. This switch allows the execution of programs with a suid bit inside the jail. This may be a security threat, use at your own risk. To activate this option, set ALLOWSUID=true.